1. Field of the Invention
This invention relates to an AFC circuit for an FM receiver circuit and an IC of the AFC circuit.
2. Description of the Prior Art
Various FM receivers are conventionally known and used practically. An exemplary one of such conventional FM receivers is shown in FIG. 3. Referring to FIG. 3, broadcast wave signals received by an antenna 1 are supplied by way of a high frequency amplifier 2 to a tuning circuit 3, by which a broadcast wave signal of an aimed frequency is selected. The selected broadcast wave signal is supplied to a mixer circuit 4, by which it is converted by frequency conversion into an intermediate frequency signal (the intermediate frequency signal has the frequency of, for example, 10.7 MHz) using a local oscillation signal from a local oscillation circuit 5. The intermediate frequency signal is supplied from the mixer 4 by way of an intermediate frequency filter 6 constituted from, for example, a ceramic filter and an intermediate frequency amplifier 7 to an FM demodulation circuit 8, by which it is demodulated into an audio signal. The local oscillation circuit 5 includes a resonance circuit 51 which operates in an interlocking relationship with the tuning circuit 3.
Further, automatic frequency control (AFC) is performed by an AFC circuit 9. In particular, a variable reactance element, i.e., a variable capacitance diode D51 in the circuit configuration shown in FIG. 3, is connected in parallel to the resonance circuit 51 by way of a pair of dc current cutting capacitors C51 and C52. The demodulation output S8 of the demodulation circuit 8 is supplied to a low-pass filter 91 which is constituted from a resistor R91 and a capacitor C91 as indicated by solid lines in FIG. 3, and a dc voltage included in the demodulation output S8 is extracted as an AFC voltage V91 from the low-pass filter 91.
The AFC voltage V91 is supplied to the variable capacitance diode D51 by way of a buffer resistor R92 while a bias voltage V92 is supplied to the variable capacitance diode D51 by way of another buffer resistor R93.
Accordingly, the capacitance of the variable capacitance diode D51 is varied in response to the AFC voltage V91, and the frequency of the local oscillation of the local oscillation circuit 5 is varied in response to a variation of the capacitance of the variable resistance diode D51 to effect automatic frequency control.
By the way, the frequency conversion of the mixer circuit 4 is, for example, in Japan, lower heterodyne conversion wherein the local oscillation frequency is lower than the frequency of the broadcast waves, but, on the contrary, for example, in the United States, it is upper heterodyne conversion wherein the local oscillation frequency is higher than the frequency of the broadcast waves.
Thus, the polarity of the AFC voltage V91 to be supplied to the variable capacitance diode D51 must be made different or inverse between an FM receiver of the lower heterodyne type and another FM receiver of the upper heterodyne type. Accordingly, in the FM receiver of FIG. 3, if it is assumed that lower heterodyne frequency is performed when the AFC voltage V91 and the bias voltage V92 are supplied in such a manner as indicated by solid lines in FIG. 3, then in order to perform upper heterodyne conversion, the AFC voltage V91 and the bias voltage V92 are supplied reversely to that described above to the variable capacitance diode D51 as indicated by broken lines in FIG. 3.
Since the relationship between the direction of the heterodyne conversion and the polarity of the AFC voltage V91 naturally applies also to the receiver circuit which is formed into an IC (integrated circuit), such IC for the FM receiver circuit is constructed, for example, in such a manner as shown in FIG. 4.
In particular, referring to FIG. 4, circuit elements and connecting lines delineated by a chain line are formed as a one-chip monolithic IC for an FM receiver circuit. The IC shown has external connection terminals T11 to T15 in the form of pins. The resonance circuit 51 is connected to the local oscillation circuit 5 of the IC by way of the terminal T11, and an audio signal from the demodulation circuit 8 of the IC is extracted to the outside of the IC by way of a resistor R81 and the terminal T15 of the IC.
Further, the demodulation output S8 of the demodulation circuit 8 is supplied to the low-pass filter 91 which is constituted from the resistor R91 of the IC and the capacitor C91 externally connected to the terminal T14, and the AFC voltage V91 is extracted from the low-pass filter 91.
Then, if the elements C92 and R92 are externally connected to the IC as indicated by solid lines, then the AFC voltage V91 from the filter 91 is supplied to the variable capacitance diode D51 by way of the signal line of an invertor 93.fwdarw.a resistor R94.fwdarw.the terminal T13.fwdarw.the resistor R92.fwdarw.the terminal T12 while the bias voltage V92 is supplied to the variable capacitance diode D51.
Accordingly, in this instance, the AFC voltage V91 is inverted in polarity by the invertor 93 and then supplied to the variable capacitance diode D51, and consequently, for example, automatic frequency control of the upper heterodyne conversion can be performed.
On the other hand, if a resistor R95 is externally connected to the IC in place of the capacitor C92 and the resistor R92 as indicated by a broken line in FIG. 4, then the AFC voltage V91 obtained at the terminal T14 is supplied to the variable capacitance diode D51 by way of the resistor R95 and the terminal T12 while the bias voltage V92 is supplied to the variable capacitance diode D51.
Accordingly, in this instance, the AFC voltage V91 is supplied to the variable capacitance diode D51 while maintaining its polarity, and consequently, automatic frequency control of the lower heterodyne conversion can be performed.
In this manner, with the AFC circuit 9 of the IC, automatic frequency control of the upper heterodyne conversion can be performed when the elements C91, C92 and R92 are externally connected, but when the elements C91 and R95 are externally connected, automatic frequency conversion of the lower heterodyne conversion can be performed. In summary, the AFC circuit 9 can cope with both of automatic frequency control of the lower heterodyne conversion and automatic frequency control of the upper heterodyne conversion by changing the parts to be externally connected and the positions of the parts.
In the case of the AFC circuit 9 of FIG. 4, however, the three terminals T12, T13 and T14, which are used only for automatic frequency control, are required, and consequently, the package of the IC occupies too much space. Also the number of parts to be externally connected is large.
Further, when an FM receiver is to be assembled using the IC, the pattern of the printed circuit board must be changed in accordance with the system of heterodyne conversion, that is, the destination of the product. Accordingly, a printed circuit board for lower heterodyne conversion and another printed circuit board for upper heterodyne conversion must necessarily be prepared and maintained in inventory.